. The clinical use of prisms; and the decentering of lenses . efes near and distant tests bothrequire a simple vertical prism, they give a falseresult unless the prism is held exactly vertical, andthe head held perfectly steady. The slightest obliq-uity of the prism introduces a lateral displacementof the false image, which we may easily mistake for a * The formula is as follows, where d is the deviatingangle of the prism, r its angle of rotation, and H the angleby which the eye deviates, and by which, therefore, the falseimage is misplaced, Tan. H = Tan. d Sin. words, the tangent of the
. The clinical use of prisms; and the decentering of lenses . efes near and distant tests bothrequire a simple vertical prism, they give a falseresult unless the prism is held exactly vertical, andthe head held perfectly steady. The slightest obliq-uity of the prism introduces a lateral displacementof the false image, which we may easily mistake for a * The formula is as follows, where d is the deviatingangle of the prism, r its angle of rotation, and H the angleby which the eye deviates, and by which, therefore, the falseimage is misplaced, Tan. H = Tan. d Sin. words, the tangent of the horizontal angle we wish tomeasure, through which the false image is misplaced, is themultiple of the sine of the angle through which the prismhas been rotated, and the tangent of the angle by which itdeflects rays of light. CLINICAL APPLICATIONS IN DIAGNOSIS. 125. latent deviation where none such exists. Toremedy this I made use, several years ago, of adouble prism, which is shewn in Fig. 57. It is com-posed of two prisms, each of 2° or 30 united by their. Fig. 57.—A double prism. bases. The patient, shutting the left eye, holdsthis prism before the right one and looks throughit at a flame, two images of which are seen, one aboveand one below the real position of the is easier than to place the prism so thatthe two images appear vertical. On opening theleft eye the flame is seen mid-way between its twoimages, and in the same vertical line if there be nolatent deviation * The double prism makes thetest more delicate than a single one, but it is nowreplaced by the still better plan of a glass rod,suggested by a faint streak of light seen uniting the * When I devised the double prism, I was not awarethat a similar one, called a bi-prism, had been used byFresnel to demonstrate the phenomena of interference. 126 PRISMS. two false images, clue to the fact, proved by , that the edge of the obtuse prism is not amathematical line, but from imperfect manufacture
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